Such conductivity sensors comprise essentially a transmitting coil and a receiving coil, which, as a rule, are embodied as toroidal coils, and which surround a traversing opening for accommodating the medium, so that, in the case of exciting the transmitting coil, a closed circuit path can form extending within the medium and passing through the transmitting and receiving coils. An inductive conductivity sensor can be described as a combination of two transformers, wherein the closed path acts as a coil with a winding belonging to both transformers. Through evaluation of the signal of the receiving coil in response to the signal of the transmitting coil, consequently, the conductivity of the medium can be ascertained.
From the state of the art, for example, from DE 10 2007 039015 A1, known inductive conductivity sensors are usually shielded against capacitive and inductive coupling with the aid of conductive supports, or housings, and shieldings of high permeability material.
From DE 10 2006 025 194 A1, a conductivity sensor is known, whose toroidal coils are formed of a plurality of first conductor sections, which extend in a plane of a multi-ply circuit card, a plurality of second conductor sections, which extend in a second plane of the circuit card, and a plurality of vias, which connect the first conductor sections with the second conductor sections, in order to form coil windings. In a form of embodiment, the coils are arranged coaxially and axially one after the other within the circuit card. Extending between the two coils within the circuit is an additional separating ply card, which can serve as a shielding ply for decoupling the coils.
Such a separating ply enables, however, only a shielding parallel to the base surfaces of the toroidal coils, however, not along its periphery. Correspondingly, toroidal coils with parallel central axes arranged next to one another, or coaxial coplanarly arranged toroidal coils, can then not be shielded sufficiently relative to one another. An encapsulation of the individual toroidal coils integrated in a circuit card, in each case, in their own housings, which shield the two coils relative to one another, introduces a number of disadvantages. Especially, in this way, a number of advantages of the integration of the conductivity sensor in a circuit card must be abandoned, such as, for example, simplified, automated manufacture or the compactness of the sensor construction.